why do most of my spam emails contain the same string of numbers?

The numbers seem random but appear in most of the stuff filtered out by my spam filter, usually in the subject line. Normally I don't care about the spam folder but It's happened enough that I notice every time that string of numbers pops up. From seemingly random accounts about random seeming things. Just curious if anyone else has had this experience or it's just some way spammers know who was sent what?